This invention in general relates to control of exposure control systems for use in photographic apparatus. More particularly, it concerns an improved apparatus facilitating calibration of the exposure control system.
Automatic exposure control systems in the photographic arts are well known. Essentially, these systems evaluate scene brightness levels of illumination for controlling exposure parameters including effective aperture size and exposure interval. Included in such a system is a photocell and operably connected thereto a scene radiation integrating and control circuit. In use, the photocell senses scene radiation incident thereon, while the circuit integrates the photocell's signal until a predetermined circuit output signal is achieved which corresponds to a particular exposure value. This output signal is effective for terminating the exposure interval of a shutter mechanism.
Such exposure control systems, in theory, should provide the designed for exposure values. In practice, however, since exposure is a function of many variable factors, there is potential for the exposure values actually produced by any such system to vary from the values designed for. These variations in exposure values often result from the cumulative effect of variations, typically within prescribed tolerances, of the optical, electronic and shutter blade systems. Thus, tolerance variations of, for example, individual electronic components making up the photocell transducer and the noted integrating and control circuitry and variations in the transmissivity properties of the optical system of the photocell can cause variations in exposure values. Obviously, correcting for such variations is of critical concern in insuring proper exposure for achieving satisfactory photographic results.
To eliminate such exposure differences between identically designed exposure control systems, it is often necessary that each such system be calibrated individually for admitting precisely the amount of light to the film plane which is required by a particular film of predetermined speed to achieve a correct exposure.
Attempts at calibrating the exposure control systems have included insertion of different neutral density filters, one at a time, into scene light intercepting relation with respect to the scene light directed to the phototransducer. A photographic exposure cycle is completed after insertion of each filter for determining the resulting exposure. This process continues until the best filter is found which operates to most closely correlate the exposure to the exposure intended for a selected film speed. One commercially available camera calibrated in the above-described manner is the SX-70 camera manufactured by Polaroid Corporation. Obviously, calibration in the above manner is not only time-consuming but expensive. Because of the large number of possible exposure variations, a large number of different filters must be maintained in stock for effecting the desired calibration. Hence, the foregoing described calibrating technique is less than satisfactory.
A significant improvement over the foregoing calibrating approach is described in U.S. Pat. No. 3,942,184 issued to I. Blinow et al. and assigned in common herewith. This approach utilized a neutral density filter which is selectively rotated into the scene light path of the photocell detector so as to vary the intensity of the scene light incident on the detector. In this manner, the output response of the detector serves to terminate an exposure interval in time to insure that the amount of scene light incident to the film plane substantially corresponds to the select film speed so as to provide the desired exposure.